Martensitic oxide dispersion strengthened alloy with enhanced high-temperature strength and creep property, and method of manufacturing the same

ABSTRACT

The present application discloses a martensitic oxide dispersion-strengthened alloy having enhanced high-temperature strength and creep properties. The alloy includes chromium (Cr) of 8 to 12% by weight, yttria (Y 2 O 3 ) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight and the balance of iron (Fe). The application also discloses a method of making the alloy.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2013-0034720 filed on Mar. 29, 2013, and Korean PatentApplication No. 2013-0164341, filed on Dec. 26, 2013, the disclosure ofwhich is incorporated herein by reference in its entirety.

BACKGROUND

1. Field

The present disclosure relates to a martensitic oxidedispersion-strengthened alloy.

2. Discussion of Related Art

Generally, when a Fe—Cr alloy obtained by adding approximately 12% byweight of chromium to iron is normalized and tempered, a tempered,martensite structure is formed. Therefore, the Fe—Cr alloy is used as astructural material for nuclear energy systems (for example,sodium-cooled fast reactors) or coal-fired power generators since it hasexcellent neutron irradiation resistance and mechanical properties at ahigh temperature. However, since such an alloy has a problem in that ithas a significantly low strength at 650° C. or higher, an oxidedispersion-strengthened alloy manufactured by dispersing an oxide, whichis stable at a high temperature, in the structural material has beendeveloped recently.

However, although conventional oxide dispersion-strengthened alloys havean advantage in that they have more excellent strength than other alloysat a high temperature, they have a problem in that they do not satisfythe design requirements.

To solve the above-described problems, research on various methods hasbeen conducted, including a method which includes adding tungsten (W) asa solid-solution hardening element to an iron (Fe)-chromium (Cr)-yttria(Y₂O₃)-based alloy and adding a minor alloying element such as vanadium(V) or niobium (Nb) to the resulting alloy mixture, wherein tungsten (W)is not softened even at a high temperature and not easily abraded due tohigh hardness (see Korean Patent Publication No. 10-2012-0118312).

However, when tungsten (W) is added as the solid-solution hardeningelement in the proposed method, tungsten (W) forms a Laves phase such asa brittle (Fe, Cr)₂W phase when it is used under a high-temperaturestress atmosphere for a long period of time. Accordingly, the proposedmethod affects the creep strain rate to be accelerated at a hightemperature, which results in manufacture of an alloy having inferiorhigh-temperature creep properties. Therefore, development of amartensitic oxide dispersion-strengthened alloy having enhancedhigh-temperature strength and creep properties is required.

The foregoing discussion in this section is to provide backgroundinformation of the invention and does not constitute an admission ofprior art.

SUMMARY

According to an aspect of the present invention, there is provided amartensitic oxide dispersion-strengthened alloy having enhancedhigh-temperature strength and creep properties, which includes chromium(Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to 0.5% by weight,carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% byweight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight, and thebalance of iron (Fe).

According to one exemplary embodiment of the present invention, titanium(Ti), zirconium (Zr) and nickel (Ni) may be included at a total contentof 0.5% by weight or less.

According to another exemplary embodiment of the present invention, themartensitic oxide dispersion-strengthened alloy may be used as amaterial for core structure parts including nuclear fuel claddings,wires, end plugs and ducts of a fast reactor.

According to another aspect of the present invention, there is provideda method of manufacturing a martensitic oxide dispersion-strengthenedalloy having enhanced high-temperature strength and creep properties.Here, the method includes:

(a) mixing an yttria (Y₂O₃) powder with a metal powder including carbon(C), iron (Fe), chromium (Cr), molybdenum (Mo), titanium (Ti), zirconium(Zr) and nickel (Ni) and manufacturing an alloy powder by mechanicallyalloying the resulting mixture;

(b) charging a can-shaped container with the mechanically alloyed alloypowder and degassing the alloy powder;

(c) manufacturing an oxide dispersion-strengthened alloy by hot-workingthe degassed alloy powder; and

(d) cold-working the hot-wrought oxide dispersion-strengthened alloy.

According to one exemplary embodiment of the present invention, in step(a) the alloy powder may include chromium (Cr) of 8 to 12% by weight,yttria (Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% byweight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of0.05 to 0.2% by weight, and the balance of iron

(Fe), wherein titanium (Ti), zirconium (Zr) and nickel (Ni) are includedat a total content of 0.5% by weight or less.

According to another exemplary embodiment of the present invention, thehot working in step (c) may be performed using at least one processselected from the group consisting of a hot isostatic pressing process,a hot forging process, a hot rolling process, a hot extrusion process,and a combination thereof.

According to still another exemplary embodiment of the presentinvention, the cold working in step (d) may be performed using at leastone process selected from the group consisting of a cold rollingprocess, a cold drawing process, a cold pilgering process, and acombination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent to those of ordinary skill in theart by describing in detail exemplary embodiments thereof with referenceto the accompanying drawings, in which:

FIG. 1 is a diagram showing the experimental results obtained bycomparing room-temperature and high-temperature strength properties of amartensitic oxide dispersion-strengthened alloy according to the presentinvention and a conventional martensitic oxide dispersion-strengthenedalloy; and

FIG. 2 is a diagram showing the experimental results obtained bycomparing high-temperature creep properties of the martensitic oxidedispersion-strengthened alloy according to the present invention and theconventional martensitic oxide dispersion-strengthened alloy.

DETAILED DESCRIPTION OF EMBODIMENTS

Certain embodiments of the present invention will be described in detailbelow with reference to the accompanying drawings. While the presentinvention is shown and described in connection with exemplaryembodiments thereof, it will be apparent to those skilled in the artthat various modifications can be made without departing from the scopeof the invention.

Unless specifically stated otherwise, all the technical and scientificterms used in this specification have the same meanings as generallyunderstood by a person skilled in the related art to which the presentinvention belongs. In general, the nomenclatures used in thisspecification and the experimental methods described below are widelyknown and generally used in the related art.

The present inventors have conducted research on a martensitic oxidedispersion-strengthened alloy having enhanced strength and creepproperties at a high temperature, and found that the martensitic oxidedispersion-strengthened alloy has more enhanced high-temperaturestrength and creep properties than conventional martensitic oxidedispersion-strengthened alloys when molybdenum (Mo) is added as asolid-solution hardening element and titanium (Ti), zirconium (Zr) andtitanium (Ti) are also added as minor alloying elements. Therefore, thepresent invention has been completed based on these facts.

In one embodiment, a martensitic oxide dispersion-strengthened alloy hasenhanced high-temperature strength and creep properties and includeschromium (Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to 0.5% byweight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of 0.2 to2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium (Zr) of0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight and thebalance of iron (Fe), wherein the titanium (Ti), zirconium (Zr) andnickel (Ni) are included at a total content of 0.5% by weight or less.

When the content of chromium (Cr) is less than 8% by weight, corrosionresistance may be degraded, whereas a martensite phase may not be easilyformed when the content of chromium (Cr) is greater than 12% by weight.Therefore, the content of chromium (Cr) is preferably in a range of 8 to12% by weight, and more preferably in a range of 9 to 11% by weight.

When the content of yttria (Y₂O₃) is less than 0.1% by weight, adispersion-strengthening effect may be poor, whereas processability maybe degraded due to an increase in dispersion-strengthening effect byresidual dispersed particles when the content of yttria (Y₂O₃) isgreater than 0.5% by weight. Therefore, the content of yttria (Y₂O₃) ispreferably in a range of 0.1 to 0.5% by weight, and more preferably in arange of 0.3 to 0.4% by weight.

When the content of molybdenum (Mo) is less than 0.2% by weight,high-temperature strength may be poorly enhanced, whereas economicfeasibility may be reduced due to the presence of a large amount ofexpensive molybdenum (Mo) when the content of molybdenum (Mo) is greaterthan 2% by weight. Therefore, the content of molybdenum (Mo) ispreferably in a range of 0.2 to 2% by weight, and more preferably in arange of 0.7 to 1.5% by weight. That is, when molybdenum (Mo) is addedinstead of tungsten (W), high-temperature strength may be more enhancedthan in conventional oxide dispersion-strengthened alloys, and formationof a Laves phase may also be inhibited under high-temperature stressconditions exposed to a neutron irradiation atmosphere, which results inenhancement of long-term creep properties.

The content of titanium (Ti) is preferably in a range of 0.01 to 0.3% byweight, and more preferably in a range of 0.1 to 0.3% by weight. Suchtitanium (Ti) is bound to yttria (Y₂O₃) in a heating process to form aY-Ti-O-based complex oxide such as Y₂Ti₂O₇ or Y₂TiO₅, which contributesto high-density fine dispersion, thereby enhancing a strength property.

The content of zirconium (Zr) is preferably in a range of 0.01 to 0.2%by weight, and more preferably in a range of 0.1 to 0.2% by weight. Suchzirconium (Zr) is also bound to yttria (Y₂O₃) in a heating process toform a Y—Zr—O-based complex oxide so that zirconium (Zr) can beuniformly dispersed in a base, and the remaining zirconium (Zr) may alsobe formed into ZrC or dissolved in a solid solution, thereby furtherenhancing a high-temperature strength property.

Nickel (Ni) is an austenite-forming element that serves to enhancestrength of a base structure due to martensite strengthening. In thiscase, the content of such nickel (Ni) is preferably in a range of 0.05to 0.2% by weight, and more preferably in a range of 0.1 to 0.2% byweight.

Meanwhile, when titanium (Ti) or zirconium (Zr) is present in a largeamount, the strength may rather be reduced due to formation of a coarseoxide such as TiO₂ or ZrO₂ and inhibition of grain refinement.Therefore, titanium (Ti), zirconium (Zr) and nickel (Ni) may be includedat a total content of 0.5% by weight or less.

According to another aspect of the present invention, the presentinvention provides a method of manufacturing a martensitic oxidedispersion-strengthened alloy having enhanced high-temperature strengthand creep properties. Here, the method includes:

(a) mixing an yttria (Y₂O₃) powder with a metal powder including carbon(C), iron (Fe), chromium (Cr), molybdenum (Mo), titanium (Ti), zirconium(Zr) and nickel (Ni) and manufacturing an alloy powder by mechanicallyalloying the resulting mixture;

(b) charging a can-shaped container with the mechanically alloyed alloypowder and degassing the alloy powder;

(c) manufacturing an oxide dispersion-strengthened alloy by hot-workingthe degassed alloy powder; and

(d) cold-working the hot-wrought oxide dispersion-strengthened alloy.

In step (a), the alloy powder is prepared by mixing the yttria (Y₂O₃)powder with the metal powder including carbon (C), iron (Fe), chromium(Cr), molybdenum (Mo), titanium (Ti), zirconium (Zr) and nickel (Ni) andmechanically alloying the resulting mixture. In this case, the metalpowder includes chromium (Cr) of 8 to 12% by weight, carbon (C) of 0.02to 0.2% by weight, molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti)of 0.01 to 0.3% by weight, zirconium (Zr) of 0.01 to 0.2% by weight,nickel (Ni) of 0.05 to 0.2% by weight and the balance of iron (Fe),wherein titanium (Ti), zirconium (Zr) and nickel (Ni) are preferablyincluded at a total content of 0.5% by weight or less. A mixed powderobtained by mixing the metal powder with 0.1 to 0.5% by weight of theyttria (Y₂O₃) powder is mechanically alloyed using a mechanical alloyingmachine such as a horizontal ball mill to prepare an alloy powder.

In step (b), the alloy powder prepared in step (a) is degassed under avacuum condition. More particularly, a can-shaped container made ofcarbon steel or stainless steel is charged with the mechanically alloyedalloy powder prepared in step (a), and sealed. Thereafter, the alloypowder is degassed at a temperature of 400 to 650° C. and a degree ofvacuum of 10⁻⁴ torr for 1 to 4 hours.

In step (c), the alloy powder degassed in step (b) is hot-wrought. Moreparticularly, an oxide dispersion-strengthened alloy is manufacturedusing at least one selected from the group consisting of a hot isostaticpressing process, a hot forging process, a hot rolling process, a hotextrusion process, and a combination thereof.

In step (d), the oxide dispersion-strengthened alloy manufactured instep (c) is cold-wrought. More particularly, the cold working may beperformed using at least one selected from the group consisting of acold rolling process, a cold drawing process, a cold pilgering process,and a combination thereof.

According to one exemplary embodiment of the present invention,martensitic oxide dispersion-strengthened alloys including chromium (Cr)of 11% by weight, yttria (Y₂O₃) of 0.35% by weight, carbon (C) of 0.15%by weight, molybdenum (Mo) of 1% by weight, titanium (Ti) of 0.1% byweight, zirconium (Zr) of 0.2% by weight, nickel (Ni) of 0.1% by weightand the balance of iron (Fe) were prepared (see Example 1), andhigh-temperature tensile strength and creep properties were comparedwith those of a conventional martensitic oxide dispersion-strengthenedalloy. As a result, the martensitic oxide dispersion-strengthened alloyaccording to the present invention was found to have an excellentstrength property (see Example 2) and creep property (see Example 3) atroom temperature and a high temperature, especially 700° C., comparedwith the conventional martensitic oxide dispersion-strengthened alloy.

Hereinafter, certain examples will be described in order to aid inunderstanding the present invention. However, it should be understoodthat the description set forth herein is merely exemplary andillustrative of exemplary embodiments for the purpose of describing thepresent invention, and is not intended to limit the present invention.

EXAMPLE 1 Manufacture of Martensitic Oxide Dispersion-Strengthened Alloy

Martensitic oxide dispersion-strengthened alloys having compositions aslisted in the following Table 1 were manufactured.

TABLE 1 Fe C Cr W Mo Ni Ti Zr Y₂O₃ Reference alloy 1 Bal. 0.15 10 2 0.250.35 Reference alloy 2 Bal. 0.15 11 2 0.1 0.25 0.35 Novel alloy Bal.0.15 11 1 0.1 0.1 0.2 0.35 Units: % by weight

That is, a high-purity source powder (Fe, Cr, Mo, Ti, Zr and Ni: a grainsize of 200 mesh or less and a purity of 99% or more) and Y₂O₃ powder (aparticle size of 50 nm or less and a purity of 99.9%) were mixed atrespective weight ratios, and then mechanically alloyed at 240 rpm for48 hours under an ultra-high purity Ar atmosphere using a horizontalball mill to manufacture an alloy powder. Thereafter, a stainless canwas charged with the alloy powder and sealed, and the alloy powder wasthen degassed at 500° C. for 3 hours under a degree of vacuum of 10⁻⁴torr, or less. The can charged with the manufactured alloy powder wassubjected to a hot isostatic pressing process under conditions of 1,150°C. and 100 MPa for 3 hours to manufacture an oxidedispersion-strengthened alloy. Subsequently, a hot rolling process wasperformed by heating the oxide dispersion-strengthened alloy at 1,150°C. for an hour. In this case, a reduction rate corresponding to onecycle of rolling was maintained at 5 to 10% of the thickness of thealloy, and the hot rolling was performed until the reduction ratereached a reduction in thickness of 80% or more by repeatedly performingseveral cycles of the hot rolling. A temperature of the alloy during arolling process was maintained in a range of 950 to 1,150° C., andintermediate heat treatment was performed at 1,150° C. for more than 5minutes. Finally, the alloy manufactured by the hot rolling was cooledin the air.

EXAMPLE 2 Comparison Test of Room-Temperature and High-TemperatureStrength Properties

The three martensitic oxide dispersion-strengthened alloys (i.e.,reference alloys 1 and 2 and the novel alloy) manufactured in Example 1were measured for yield strength (YS), ultimate tensile strength (UTS)and total elongation (TE) at room temperature and 700° C. The resultsare shown in FIG. 1.

As shown in FIG. 1, it could be seen that the reference alloy 1 to whichtungsten (W) and titanium (Ti) were added had yield strengths of 748 MPaand 195 MPa at room temperature and 700° C., respectively, and thereference alloy 2 to which nickel (Ni) was further added at a content of0.1% by weight had yield strengths of 1,309 MPa and 162 MPa at roomtemperature and 700° C., indicating that the reference alloy 2 hadenhanced tensile strength, compared with the reference alloy 1. Inparticular, it could be seen that the alloy (i.e., the novel alloy)according to the present invention in which tungsten (W) was replacedwith molybdenum (Mo) and to which titanium (Ti), zirconium (Zr) andnickel (Ni) were added together had significantly enhanced tensilestrength at room temperature and a high temperature.

From these results, it could be seen that the martensitic oxidedispersion-strengthened alloy according to the present invention hadenhanced tensile strength at room temperature and a high temperature,especially around 700° C., compared with the conventional martensiticoxide dispersion-strengthened alloy.

Example 3 Comparison Test of High-Temperature Creep Property

A creep test was performed at 700° C. on the three martensitic oxidedispersion-strengthened alloys prepared in Example 1. The results areshown in FIG. 2.

As shown in FIG. 2, it could be seen that the alloy (i.e., a novelalloy) according to the present invention in which tungsten (W) wasreplaced with molybdenum (Mo) and to which titanium (Ti), zirconium (Zr)and nickel (Ni) were added together had a significantly increased creeprupture time under stresses of 100 and 120 MPa, compared with thereference alloys 1 and 2 containing tungsten (W) and titanium (Ti).

From these results, it could be seen that the martensitic oxidedispersion-strengthened alloy according to the present invention had amore excellent high-temperature creep property than the conventionalmartensitic oxide dispersion-strengthened alloy.

The martensitic oxide dispersion-strengthened alloy according to thepresent invention includes chromium (Cr) of 8 to 12% by weight, yttria(Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight,molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% byweight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to0.2% by weight and the balance of iron (Fe). Here, titanium (Ti),zirconium (Zr) and nickel (Ni) are included at a total content of 0.5%by weight or less. Therefore, the martensitic oxidedispersion-strengthened alloy of the present invention has excellentstrength and creep properties at a high temperature, especially around700° C., and thus is expected to be able to be effectively used as amaterial for core structure parts, including nuclear fuel claddings,wires, end plugs and ducts of a fast reactor such as a sodium-cooledfast reactor.

It will be apparent to those skilled in the art that variousmodifications can be made to the above-described exemplary embodimentsof the present invention without departing from the scope of theinvention. Thus, it is intended that the present invention covers allsuch modifications provided they come within the scope of the appendedclaims and their equivalents.

What is claimed is:
 1. A martensitic oxide dispersion-strengthened alloycomprising: chromium (Cr) of 8 to 12% by weight, yttria (Y₂O₃) of 0.1 to0.5% by weight, carbon (C) of 0.02 to 0.2% by weight, molybdenum (Mo) of0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% by weight, zirconium(Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to 0.2% by weight,and the balance of iron (Fe).
 2. The martensitic oxidedispersion-strengthened alloy of claim 1, wherein the sum of titanium(Ti), zirconium (Zr) and nickel (Ni) in the alloy is 0.5% by weight orless with reference to the total weight of the alloy.
 3. The martensiticoxide dispersion-strengthened alloy of claim 1, wherein the martensiticoxide dispersion-strengthened alloy is shaped to form at least one of aa nuclear fuel cladding, a wire, an end plug and a duct of a fastreactor.
 4. A method of manufacturing a martensitic oxidedispersion-strengthened alloy having high-temperature strength and creepproperties, the method comprising: mixing yttria (Y₂O₃) powder withpowder of carbon (C), iron (Fe), chromium (Cr), molybdenum (Mo),titanium (Ti), zirconium (Zr) and nickel (Ni) to provide alloy powder;charging alloy powder in a container and degassing the alloy powder;hot-working the degassed alloy powder to produce an oxidedispersion-strengthened alloy; and cold-working the hot-wrought oxidedispersion-strengthened alloy.
 5. The method of claim 4, wherein thealloy powder comprises: chromium (Cr) of 8 to 12% by weight, yttria(Y₂O₃) of 0.1 to 0.5% by weight, carbon (C) of 0.02 to 0.2% by weight,molybdenum (Mo) of 0.2 to 2% by weight, titanium (Ti) of 0.01 to 0.3% byweight, zirconium (Zr) of 0.01 to 0.2% by weight, nickel (Ni) of 0.05 to0.2% by weight and the balance of iron (Fe), wherein the sum of titanium(Ti), zirconium (Zr) and nickel (Ni) in the alloy powder is 0.5% byweight or less with reference to the total weight of the alloy powder.6. The method of claim 4, wherein the hot working is performed using atleast one process selected from the group consisting of a hot isostaticpressing process, a hot forging process, a hot rolling process, a hotextrusion process, and a combination thereof.
 7. The method of claim 4,wherein the cold working is performed using at least one processselected from the group consisting of a cold rolling process, a colddrawing process, a cold pilgering process, and a combination thereof.